[W126 Coupe] 6-Speed and Replacement Diffs

[JR560SEC at aol.com]

Greetings, all--------------
 
With all the discussion in recent days regarding replacement differentials,  
I'll share my decision-making on my 6-speed manual transmission conversion,  
in the hope of stirring some interesting discussion and finding some other  
like-minded enthusiasts.  My hope is to enjoy performance and mileage that  far 
exceeds the stock vehicle, together with the opportunity to shift my own  gears 
in a W126 coupe--a rare opportunity, bordering on unique.
 
I'm preparing to perform a 6-speed conversion on my 1990 560 SEC (USA) with  
Euro 5.6 engine.  I've accumulated the "miscellaneous" parts required  for the 
conversion, including W126 manual-transmission pedals and R107 350  SL 
manual-transmission hardware, from flywheel to drive shaft to hydraulic  clutch 
pieces.The last big decision to be made is which type of transmission to  use.
 
On one hand, the Tremec (Borg-Warner) transmission will  provide gear ratios 
(1-6) of 3.36, 2.07, 1.35, 1.00, 0.80, and 0.62.   Besides 1st gear, which 
also is available in 2.97, there are no options on gear  ratios.
 
This transmission is impractical with the original USA gears, being a  step 
backward, performance-wise.  Using a 3.27 final-drive ratio  instead, speeds in 
gears 1 through 4, at 6,000 RPM are:  42, 68,  104, and 140.  Thus, 0-to-60 
requires 1 gear change and the quarter mile  requires two gear changes.  
6th-gear gives 2,650 RPM at 100 MPH, and 1,850  RPM at 70 MPH.  First-gear  3.36 x 
final-drive 3.27 gives 10.99,  better than both the Euro automatic transmission 
1st-gear (3.87) x Euro  SEC-standard 2.65 final-drive (10.26) and the USA 
auto 1st-gear x USA  SEC-standard 2.47 final-drive (9.09).
 
On the other hand, the Richmond Over Drive 6-speed has  numerous options for 
gear ratios.  My favored ratios are (1-6):  4.41,  2.75, 2.13, 1.74, 1.00, and 
0.76.  This compares closely to an old-school  American muscle-car 
close-ratio 4-speed (Think 1969 Camaro Z28 with 4.11  rear), plus two additional VERY 
tall cruising gears.
 
This transmission's ratios work great with the original 2.47 USA rear-end  
ratio.  Speeds in gears 1 through 4, at 6,000 RPM are:  42, 68, 87,  and 107.  
Thus, 0-to-60 requires 1 gear change and the quarter mile  requires three gear 
changes.  Shifting from 4th to 5th at 107 MPH  drops RPMs from 6000 to 3450, 
quite a large leap.  6th gear gives  2,450 RPM at 100 MPH, and 1,720 RPM at 70 
MPH.  First-gear  4.41  x final-drive 2.47 gives 10.89, comparable to the 
Tremec 6-speed and better  than either USA- or Euro-spec first gears.
 
The cost of the Tremec transmission is about $1,500.00, and the Richmond is  
roughly twice as much.  The Tremec transmission is marginal but acceptable  
for the Euro engine's horsepower and torque, while the Richmond would be happy  
with supercharged M117s, radical M119s, or even the LS7.  The Richmond also  
is a little noisier, but smooth-shifting nonetheless.
 
 
It looks like the Richmond is the obvious choice.  Otherwise, I would  need 
to find a 1986-91 SEC-specific 3.27 limited-slip rear, with ABS sensor  for 
about $1,500.00 or less.  In addition, if the rear end ever  failed, I'd be back 
to the drawing board.  Keeping the garden-variety 2.47  rear means I'll never 
be short of spare parts.
 
If anybody is interested in the conversion process, keep in  touch.  Please 
also keep in touch if you have your own conversion underway,  and we can swap 
lessons learned.  I'm certain there will be many.
 
Regards,
 
Jim Rothrock

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